Syphilis is a disease caused by Treponema pallidum (hereinafter sometimes referred to briefly as Tp). The diagnosis of syphilis is generally made by an immunoassay of anti-Tp antibody in the blood. The surface of Tp cell has a large number of membrane antigens and the above mentioned immunoassay utilizes the antigen-antibody reaction between the membrane antigen and the anti-Tp antibody in a blood specimen. The Tp membrane antigen for use in the immunoassay today is prepared by innoculating Tp into rabbit testis and disrupting the harvested viable cells. However, since Tp is cultured using rabbit testis, the sensitivity and reproducibility of this assay are low owing to contamination with impurities and, it is difficult to provide a large quantity of Tp.
To overcome these disadvantages, it has been proposed to clone a gene coding for the Tp membrane antigen, mass-produce said membrane antigen by biotechnology and use it in the above mentioned immunoassay. For example, Japanese Kohyo publication Hei-2-500403 describes a technology which comprises preparing a Tp antigen having a molecular weight (MW) of 47 kDa by biotechnology and assaying the anti-Tp antibody immunologically using this antigen.
The technology disclosed in Japanese Kohyo publication Hei-2-500403 enables the immunoassay of anti-Tp antibody, but it would benefit the diagnosis of syphilis should the anti-Tp antibody be assayed with improved sensitivity.
Therefore, this invention has for its object to provide a method for assay of anti-Tp antibody which enables the assay of anti-Tp antibody with improved sensitivity as compared with the conventional method.